Method for holographic mastering and replication

ABSTRACT

A method for producing holograms with a multiplicity of holographic prescriptions from a single master is provided. A multiplicity of holographic substrates each having a first hologram is stacked on a second holographic recording medium substrate. The first hologram is designed to diffract light from a first direction into a second direction. When expose to illumination from the first direction zero order and diffracted light from each first hologram interfere in the second holographic recording medium substrate forming a second hologram. The second hologram is then copied into a third holographic recording medium substrate to provide the final copy hologram.

PRIORITY CLAIM

This application claims the priority of U.S. Provisional PatentApplication No. 61/999,867 with filing date 8 Aug. 2014 entitled METHODFOR HOLOGRAPHIC MASTERING AND REPLICATION.

Cross Reference to Related Applications

The following patent applications are hereby incorporated by referenceherein in their entireties: PCT/GB2013/000273 entitled ELECTRICALLYCONTROLLABLE MASTER HOLOGRAM FOR CONTACT COPYING; U.S. Pat. No.:8,233,204 entitled OPTICAL DISPLAYS; U.S. patent application Ser. No.:13/844,456 entitled WIDE FIELD OF VIEW COLOR DISPLAY; U.S. patentapplication Ser. No.: 13/869,866 entitled APERTURE SAMPLING FOR DUALAXIS SAMPLING; U.S. patent application Ser. No.: 13/504,566 entitledCOMPACT EDGE ILLUMINATED EYEGLASS DISPLAY; PCT Application No.:US2008/001909, entitled LASER ILLUMINATION DEVICE; PCT Application No.:US2006/043938, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENTDISPLAY; PCT Application No.: PCT/GB2012/000680, entitled IMPROVEMENTSTO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES;

U.S. Pat. No. 6,115,152 entitled HOLOGRAPHIC ILLUMINATION SYSTEM; andU.S. Pat. No.: 6,323,970 entitled METHOD OF PRODUCING SWITCHABLEHOLOGRAMS.

BACKGROUND OF THE INVENTION

The present invention relates to holography and more particularly to animproved method for mastering and replicating holograms.

Replication of holograms is normally carried out by preparing a masterhologram of the desired prescription which is then copied into anotherholographic recording material using a contact process. The master isusually made using a classical two-beam holographic recording systemcomprising an object beam and a reference beam. However, the mastercould itself be a copy of another master. In the case of a transmissionhologram the copying process is based on interfering the diffracted andzero order beams produced by master to form a grating within the copyhologram material. Subject to processing variations such as shrinkagethe holographic pattern or grating formed in the copy should beidentical to the one in the master. This procedure may be used in massproduction roll-to-roll processes. The principles of holographicreplication and industrial processes for the mass production ofholograms are well documented in the literature.

The optical design benefits of diffractive optical elements (DOEs) arewell known, including unique and efficient form factors and the abilityto encode complex optical functions such as optical power and diffusioninto thin layers. Bragg gratings (also commonly termed volume phasegrating or holograms), which offer the highest diffraction efficiencies,have been widely used in devices such as Head Up Displays. An importantclass of Bragg grating devices is known as a Switchable Bragg Grating(SBG). An SBG is a diffractive device formed by recording a volume phasegrating, or hologram, in a polymer dispersed liquid crystal (PDLC)mixture. Typically, SBG devices are fabricated by first placing a thinfilm of a mixture of photopolymerizable monomers and liquid crystalmaterial between parallel glass plates or substrates. Techniques formaking and filling glass cells are well known in the liquid crystaldisplay industry. One or both glass substrates support electrodes,typically transparent indium tin oxide films, for applying an electricfield across the PDLC layer. A volume phase grating is then recorded byilluminating the liquid material with two mutually coherent laser beams,which interfere to form the desired grating structure. During therecording process, the monomers polymerize and the HPDLC mixtureundergoes a phase separation, creating regions densely populated byliquid crystal micro-droplets, interspersed with regions of clearpolymer. The alternating liquid crystal-rich and liquid crystal-depletedregions form the fringe planes of the grating. The resulting volumephase grating can exhibit very high diffraction efficiency, which may becontrolled by the magnitude of the electric field applied across thePDLC layer. When an electric field is applied to the hologram viatransparent electrodes, the natural orientation of the LC droplets ischanged causing the refractive index modulation of the fringes to reduceand the hologram diffraction efficiency to drop to very low levels. Notethat the diffraction efficiency of the device can be adjusted, by meansof the applied voltage, over a continuous range from near 100%efficiency with no voltage applied to essentially zero efficiency with asufficiently high voltage applied.

SBGs may be used to provide transmission or reflection gratings for freespace applications. SBGs may be implemented as waveguide devices inwhich the HPDLC forms either the waveguide core or an evanescentlycoupled layer in proximity to the waveguide. In one particularconfiguration to be referred to here as Substrate Guided Optics (SGO)the parallel glass plates used to form the HPDLC cell provide a totalinternal reflection (TIR) light guiding structure. Light is “coupled”out of the SBG when the switchable grating diffracts the light at anangle beyond the TIR condition. SGOs are currently of interest in arange of display and sensor applications. Although much of the earlierwork on HPDLC has been directed at reflection holograms transmissiondevices are proving to be much more versatile as optical system buildingblocks and tend to be much easier to fabricate.

Typically, the HPDLC used in SBGs comprise liquid crystal (LC),monomers, photoinitiator dyes, and coinitiators. The mixture frequentlyincludes a surfactant. The patent and scientific literature containsmany examples of material systems and processes that may be used tofabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5, 942,157 bySutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. both filingsdescribe monomer and liquid crystal material combinations suitable forfabricating SBG devices.

One of the known attributes of transmission SBGs is that the LCmolecules tend to align normal to the grating fringe planes. The effectof the LC molecule alignment is that transmission SBGs efficientlydiffract P polarized light (ie light with the polarization vector in theplane of incidence) but have nearly zero diffraction efficiency for Spolarized light (ie light with the polarization vector normal to theplane of incidence. Transmission SBGs may not be used at near-grazingincidence as the diffraction efficiency of any grating for Ppolarization falls to zero when the included angle between the incidentand reflected light is small. A glass light guide in air will propagatelight by total internal reflection if the internal incidence angle isgreater than about 42 degrees. Thus waveguide transmission SBGs may beused if the internal incidence angles are in the range of 42 to about 70degrees, in which case the light extracted from the light guide by thegratings will be predominantly p-polarized.

Normally SBGs diffract when no voltage is applied and are switching intotheir optically passive state when a voltage is application other times.However SBGs can be designed to operate in reverse mode such that theydiffract when a voltage is applied and remain optically passive at allother times. Methods for fabricating reverse mode SBGs are disclosed ina United States Provisional Patent Application No. 61/573,066. withfiling date 24 Aug. 2011 by the present inventors entitled IMPROVEMENTSTO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND which isincorporated by reference herein in its entirety. The same referencealso discloses how SBGs may be fabricated using flexible plasticsubstrates to provide the benefits of improved ruggedness, reduce weightand safety in near eye applications.

The present invention is motivated by the requirement to record SBGs ofdiffering optical prescriptions for use in image transmitting waveguidescurrently being designed for Head Up Displays (HUDs) and Head MountedDisplays (HMDs). The holograms may configured as stacks U.S. Pat. No.:8,233,204 entitled OPTICAL DISPLAYS U.S. patent application Ser. No.:13/844,456 entitled WIDE FIELD OF VIEW COLOR DISPLAY; or tessellated insingle layers as disclosed in U.S. patent application Ser. No.:13/869,866 entitled APERTURE SAMPLING FOR DUAL AXIS SAMPLING. In suchapplications the holograms are used to tile a field of view (FOV) spaceand/or increase the size of the exit pupil. For large FOV full colourdisplays the number of holographic prescriptions can be high as the FOVof a holographic element is limited by diffraction efficiency angularbandwidth. Since the cost of fabricating masters using conventionalholographic interferometry or ruling processes is currently very highthis can make the manufacture of large FOV displays very expensive.Exemplary holographic masters and replicas thereof) are provided bycompanies such as Holographix Inc. (MA). Typically, masters are surfacerelief components fabricated using holographic, binary grating etchingor mechanical ruling processes. Desirably, a mastering and replicationprocess for large FOV holographic waveguides should provide a range ofoptical prescriptions spanning the required FOV space using a minimalnumber of master components. Ideally this should be accomplished withjust one master. Applications such as HMDs and HUDs typically demandtight control of the diffraction efficiency and geometrical opticalcharacteristics of the replicated holograms. In particular there is aneed for precise control of the intensities of the diffracted and zeroorder beams. Currently available holographic mastering process sufferfrom the problem that the relative intensities of the diffracted andzero orders cannot be controlled to better than ±5%. As disclosed in aco-pending patent application PCT/GB2013/000273 the inventors havediscovered that a perfect copy can be made if the master hologram is“over-modulated” by a small amount. Over-modulation in this contextmeans that the refractive index modulation of the hologram is a littleabove that required to achieve the desired beam ratio. The next step isto separately attenuate the master beams to bring them to the desiredratio. Typically we require 50/50 or 1:1. However, the inventors havefound that making a perfect master with the appropriate level ofover-modulation, which is typically 5-10%, is very difficult inpractice. To the best of the inventors' knowledge the required levels ofindex modulation control have not been achieved using conventionalholographic recording processes using currently available holographicrecording materials such as photopolymers and Photo Thermo Refractive(PTR) materials. Desirably a holographic mastering process shouldinclude methods for controlling the hologram modulation.

There is requirement for an efficient and cost-effective method forreplicating holograms with a multiplicity of holographic prescriptionsfrom a single master.

SUMMARY OF THE INVENTION

There is provided a efficient and cost-effective method for replicatingholograms with a multiplicity of holographic prescriptions from a singlemaster.

The objects of the invention are achieved in a first embodiment in whichthere is provided a method for mastering and replicating holograms, themethod comprising:

a) providing N substrates each containing a first hologram fordiffracting incident light from a first direction into diffracted lightin a second direction; providing a second holographic recording medium;and providing a third holographic recording medium;b) stacking in sequence the first holograms 1-N onto the secondholographic recording medium;c) illuminating external surface of the first hologram N with light of afirst polarization in a first direction;d) the first holograms 1-N diffracting the light into zero order lightin the first direction and diffracted light in the second direction;e) the first direction light and the second direction light interferingin the second holographic recording medium to form a second hologram;f) placing the second hologram in contact with the third holographicrecording medium;g) illuminating external surface of the second hologram with light inthe first direction;h) the second hologram diffracting the light into zero order light inthe first direction and diffracted light in the second direction;i) the diffracted and first order light interfering in the thirdholographic recording medium to form a third hologram.

In one embodiment of the invention steps c) to i) are repeated for amultiplicity of values of the first and second directions. The first andsecond directions are limited by the diffraction efficiency angularbandwidth of said first hologram.

In one embodiment of the invention the first holograms 1,N are providedby the steps of : configuring a laser holographic recording apparatus toform a first recording beam in the first direction and a secondrecording beams in the second direction; providing N substrates eachcontaining a first holographic medium; and the first and second beamsinterfering within each the first holographic medium substrate to formthe first hologram in each the substrate.

In one embodiment the first holograms 1,N are surface relief structures.In one embodiment the first holograms 1,N are binary structures.

In one embodiment step a) further comprises providing a half wave plate(HWP) and step c) further comprises disposing the HWP between theholographic recording medium substrate and the first hologram stack. Ina further embodiment step a) further comprises providing a linearpolarizer and in step c) further comprises disposing the linearpolarizer between the HWP and the first hologram stack.

In one embodiment the first holographic recording medium is a HPDLC forrecording a SBG, the second holographic recording medium is aholographic photopolymer and the third holographic recording medium is aholographic photopolymer.

In one embodiment of the invention the third hologram is a copy of thesecond hologram and the second hologram is a copy of the first hologram.

In one embodiment of the invention the third holographic recordingmedium comprises HPDLC material components for forming one of a forwardmode SBG or a reverse mode SBG.

In one embodiment of the invention the zero order light and diffractedlight in at least one step d) and step i) have power substantially inthe ratio of 1:1.

In one embodiment of the invention the third holographic recordingmedium has a substrate fabricated from optical plastic.

In one embodiment of the invention the second hologram and the thirdholographic recording medium are separated by an air gap. In oneembodiment the second hologram and the third holographic recordingmedium are in contact.

In one embodiment of the invention the third holographic recordingmedium forms part of a mechanically translatable continuous lamina.

In one embodiment of the invention there is further provided a voltagegenerator for applied a voltage across at least one of the secondhologram and the third holographic recording medium. The applied voltagevaries the refractive index modulation of at least one of the secondhologram and the third hologram during steps g) to i).

In one embodiment of the invention the second holographic recordingmedium is one of a photo thermal refractive or holographic photopolymer,a forward mode HPDLC mixture or a reverse mode HPDLC mixture. In oneembodiment of the invention the third holographic recording medium isone of a photo thermal refractive or photopolymer, a forward mode HPDLCmixture or a reverse mode HPDLC mixture.

In one embodiment of the invention the diffracting thickness of thefirst hologram is less than or equal to 1 micron. In one embodiment ofthe invention the diffracting thickness of the first hologram is lessthan or equal to 2 micron.

In one embodiment of the invention there is provided a method ofmastering and replicating holograms, the method comprising:

a) providing a laser apparatus for forming a first recording beam in afirst direction and a second recording beams in a second direction; Nsubstrates each containing a first HPDLC mixture; a holographicphotopolymer ; a copy holographic substrate containing a second HPDLCmixture; a HWP; and a linear polarizer;b) the first and second beams interfering within each the first HPDLCmixture to form a first hologram in each substrate;c) stacking in sequence the linear polarizer, HWP and first holograms1-N onto the second holographic photopolymer;d) illuminating external surface of the hologram N with light of a firstpolarization in the first direction;e) the first holograms 1-N diffracting the light into zero order lightin the first direction and diffracted light in the second direction;f) the HWF rotating the incident light polarization through ninetydegrees into a second polarization;g) the linear polarizer removing residual first polarization light;h) the first direction light and the second direction light interferingin the holographic photopolymer to form a second hologram;i) placing the second hologram in contact with the copy holographicsubstrate;j) illuminating external surface of the second hologram with light ofthe second polarization in the first direction;k) the second hologram diffracting the light into zero order light inthe first direction and diffracted light in the second direction;l) the diffracted and first order light interfering in the copyholographic substrate to form a third hologram.

In one embodiment of the invention steps d) to l) are repeated for amultiplicity of values of the first and second directions, wherein thefirst and second directions are limited by the diffraction efficiencyangular bandwidth of the first hologram.

In one embodiment of the invention the first polarization isP-polarization and the second polarization is S-polarization.

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, wherein like index numerals indicate like parts.For purposes of clarity, details relating to technical material that isknown in the technical fields related to the invention have not beendescribed in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a process for mastering andreplicating holograms in one embodiment of the invention.

FIG. 2A is a cross section view of a stack of first holograms in oneembodiment.

FIG. 2B is a schematic cross section view of a stack comprising firstholograms, a linear polarizer, a half wave plate and second holographicrecording medium in one embodiment.

FIG. 2C is a schematic cross section view of a second hologram incontact with a third holographic recording medium in one embodiment ofthe invention.

FIG. 3A is a schematic cross section view of a stack of first hologramsin one embodiment of the invention.

FIG. 3B is a schematic cross section view of a stack comprising a stackof first holograms, a linear polarizer, a half wave plate and secondholographic recording medium in one embodiment of the invention.

FIG. 3C is a schematic cross section view of a second hologram incontact with a third holographic recording medium in one embodiment ofthe invention.

FIG. 4A is a schematic cross section view of a first hologram in oneembodiment of the invention.

FIG. 4B is a schematic cross section view of a stack of first hologramsshowing ray paths in one embodiment of the invention.

FIG. 4C is a schematic cross section view of a stack comprising a stackof first holograms, a linear polarizer, a half wave plate and secondholographic recording medium showing ray paths for a first illuminationdirection in one embodiment of the invention.

FIG. 4D is a schematic cross section view of a stack comprising a stackof first holograms, a linear polarizer, a half wave plate and secondholographic recording medium showing ray paths for a second illuminationdirection in one embodiment of the invention.

FIG. 4E is a schematic cross section view of a second hologram incontact with a third holographic recording medium showing ray paths inone embodiment of the invention.

FIG. 5 is a schematic illustration illustrating the use of an electricfield to control the refractive index modulations of the first hologramsduring the recording of the second hologram in one embodiment of theinvention

FIG. 6 is a schematic illustration illustrating the use of an electricfield to control the refractive index modulations of the second hologramduring the recording of the third hologram in one embodiment of theinvention

FIG. 7 is a schematic illustration of an optical arrangement forrecording on of the first holograms in one embodiment of the invention.

FIG. 8 is a flowchart illustrating a process for mastering andreplicating holograms in one embodiment of the invention.

FIG. 9 is a flowchart illustrating a process for mastering andreplicating holograms in one embodiment of the invention.

FIG. 10 is a flowchart illustrating a process for mastering andreplicating holograms in one embodiment of the invention.

FIG. 11 is a flowchart illustrating a process for mastering andreplicating holograms in one embodiment of the invention.

FIG. 12 is a flowchart illustrating a process for mastering andreplicating holograms in one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described by way of example only withreference to the accompanying drawings. It will apparent to thoseskilled in the art that the present invention may be practiced with someor all of the present invention as disclosed in the followingdescription. For the purposes of explaining the invention well-knownfeatures of optical technology known to those skilled in the art ofoptical design and visual displays have been omitted or simplified inorder not to obscure the basic principles of the invention. Unlessotherwise stated the term “on-axis” in relation to a ray or a beamdirection refers to propagation parallel to an axis normal to thesurfaces of the optical components described in relation to theinvention. In the following description the terms light, ray, beam anddirection may be used interchangeably and in association with each otherto indicate the direction of propagation of light energy alongrectilinear trajectories. Parts of the following description will bepresented using terminology commonly employed by those skilled in theart of optical design. The term “grating” may be used to describe ahologram. It should also be noted that in the following description ofthe invention repeated usage of the phrase “in one embodiment” does notnecessarily refer to the same embodiment.

The present invention provides a method for producing holograms with amultiplicity of holographic prescriptions from a single master. Themaster which will be described as a first hologram is characterised by awide angular bandwidth. Desirably, the first hologram also has largeindex modulation. This allows a wide range of input and diffracted beamangles to be generated by the first hologram. For each set of input anddiffracted beam angle an intermediate master (second) hologram isrecorded. The resulting set of intermediate master (second) hologramsmay then be used to contact copy the hologram into the desired copymedium to provide a copy (third) hologram. A wide bandwidth hologramwill have .a small thickness which results have relatively lowdiffraction efficiency. In the proposed method the problem of lowdiffraction efficiency is overcome by stacking a multiplicity ofholographic substrates each containing the first hologram. This stack isthen overlaid on stacked on a second holographic recording mediumsubstrate. The first hologram is designed to diffract light from a firstdirection into a second direction. When exposed to illumination from thefirst direction zero order and diffracted light from each (first)hologram in the stack interfere in the second holographic recordingmedium substrate forming a second hologram. The second hologram is thencopied into a third holographic recording medium substrate to providethe final copy hologram. The invention may be used to master andreplicate any type of hologram in any type of holographic recordingmaterial. The invention may be used to master and replicated passive orswitchable holograms. The holograms may be single elements or switchablearrays as described in PCT/GB2013/000273. Voltages may be applied acrossthe second hologram to control the index modulation and hence fine tunebeam ratios during the final contact copying stage. Voltages may also beapplied across the first holograms during the recording of the secondhologram.

In one embodiment of the invention there is provided a method formastering and replicating holograms, the method comprising:

a) providing N substrates each containing a first hologram fordiffracting incident light from a first direction into diffracted lightin a second direction; providing a second holographic recording medium;and providing a third holographic recording medium;b) stacking in sequence the first holograms 1-N onto the secondholographic recording medium;c) illuminating external surface of the first hologram N with light of afirst polarization in a first direction;d) the first holograms 1-N diffracting the light into zero order lightin the first direction and diffracted light in the second direction;e) the first direction light and the second direction light interferingin the second holographic recording medium to form a second hologram;f) placing the second hologram in contact with the third holographicrecording medium;g) illuminating external surface of the second hologram with light inthe first direction;h) the second hologram diffracting the light into zero order light inthe first direction and diffracted light in the second direction;i) the diffracted and first order light interfering in the thirdholographic recording medium to form a third hologram.

In one embodiment of the invention steps c) to i) are repeated for amultiplicity of values of the first and second directions. The first andsecond directions are limited by the diffraction efficiency angularbandwidth of said first hologram.

A method of replicating a hologram in one embodiment of the invention inaccordance with the basic principles of the invention is shown in theflow diagram in FIG. 1. Referring to the flow diagram, we see that themethod comprises the following steps:

At step 2001 provide N hologram substrates each containing a firsthologram having construction angles in first and second directions; asecond holographic recording medium; and a third holographic recordingmedium.At step 2002 stack hologram substrates 1-N onto the second holographicrecording mediumAt step 2003 illuminate hologram substrates (1-N) with light in thefirst directionAt step 2004 hologram substrates (1-N) provide 0-order light in thefirst direction and diffracted light in the second direction.At step 2005 the first and second direction light interferes to form asecond hologram in the second holographic recording medium.At step 2006 place the second hologram in contact with the thirdholographic recording medium.At step 2007 illuminate the second hologram with light in the firstdirection.At step 2008 the second hologram provides 0-order light in the firstdirection and diffracted light in the second direction.At step 2009 the first and second direction light interferes to form thethird hologram.

Note that in terms of defining the holographic prescription a hologramhaving construction angles in the first and second directions isequivalent to the same hologram diffracting incident light from a firstdirection into diffracted light in a second direction.

FIG. 2A shows the stack of first holograms labelled by 11-12. FIG. 2Bshows the stack of first holograms overlaying of the second holographicrecording medium 40. FIG. 2C shows the second hologram overlaying thethird holographic recording medium.

In one embodiment of the invention the first holograms 1,N are providedby the steps of firstly, configuring a laser holographic recordingapparatus to form a first recording beam in the first direction and asecond recording beams in the second direction; secondly, providing Nsubstrates each containing a first holographic medium; and, thirdly, thefirst and second beams interfering within each the first holographicmedium substrate to form the first hologram in each substrate. Thepresent invention does not assume that any particular holographicrecording process or HPDLC material is used to fabricate the firstholograms. Any of the processes and material systems currently used tofabricate SBGs may be used such as for example the ones disclosed inU.S. Pat. No. 5, 942,157 by Sutherland, and U.S. Pat. No. 5,751,452 byTanaka. The master may be recorded using currently available industrialprocesses such as the ones provided by companies such as Holographix LLC(MA). Ideally, the master would be recorded using remote computercontrolled equipment, which by removing human presence eliminatesvibrations and thermal variations that may adversely affect the qualityof the recording process. Ideally, the master recording laboratoryshould be protected from vibrations from external disturbances.Desirably, the master hologram recording equipment will provide activefringe stabilization.

In the preferred embodiments the first hologram and third (copy)holograms are SBGs. In one embodiment the SBGs are reverse mode such thehologram diffracts when a voltage is applied and remains opticallypassive at all other times. A reverse mode SBG will provide lower powerconsumption. A reverse mode HPDLC and methods for fabricating reversemode SBG devices is disclosed in U.S. Provisional Patent Application No.61/573,066. with filing date 24 Aug. 2011 by the present inventorsentitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTALMATERIALS AND which is incorporated by reference herein in its entirety.Ultimately, the inventors aim to make replica SBGs with plasticsubstrates and flexible transparent conductive coatings (to replaceITO). Plastic SBG technology suitable for the present invention is alsodisclosed in U.S. Provisional Patent Application No. 61/573,066. Areverse mode SBG is more ideally suited to mastering as it avoids thedegradation of SBG material that occurs with UV recording.Advantageously, the SBGs will used thin flexible glass substrates suchas the ones developed by Corning and Schott driven by the touch paneland smart phone industries.

In one embodiment of the invention the first holograms 1,N are surfacerelief structures such as binary structures. Such holograms wouldtypically require index matching layers between the hologram layers

In one embodiment of the invention step a) further comprises providing ahalf wave plate (HWP) and step c) further comprises disposing the HWPbetween the holographic recording medium substrate and the firsthologram stack. In a further embodiment step a) further comprisesproviding a linear polarizer and in step c) further comprises disposingthe linear polarizer between the HWP and the first hologram stack.

In one embodiment of the invention the first holographic recordingmedium is a HPDLC for recording a SBG, the second holographic recordingmedium is a holographic photopolymer and the third holographic recordingmedium is a holographic photopolymer. In one embodiment of the inventionthe third hologram is copy of the second hologram and the secondhologram is a copy of the first hologram. In one embodiment of theinvention the third holographic recording medium comprises HPDLCmaterial components for forming one of a forward mode SBG or a reversemode SBG. In one embodiment of the invention the zero order light anddiffracted light in at least one step d) and step i) have powersubstantially in the ratio of 1:1. In one embodiment of the inventionthe third holographic recording medium has a substrate fabricated fromoptical plastic. In one embodiment of the invention the second hologramand the third holographic recording medium are separated by an air gap.In one embodiment of the invention the second hologram and the thirdholographic recording medium are in contact. In one embodiment of theinvention the third holographic recording medium forms part of amechanically translatable continuous lamina.

In one embodiment of the invention there is further provided a voltagegenerator for applied a voltage across at least one of the secondhologram and the third holographic recording medium according to theprinciples disclosed in PCT/GB2013/000273 entitled ELECTRICALLYCONTROLLABLE MASTER HOLOGRAM FOR CONTACT COPYING. The voltage varies therefractive index modulation of at least one of the second hologram andthe third during steps g) to i). FIG. 5 illustrates the application of avoltage across the first holograms using the voltage generator 15 viathe connectors 16. FIG. 6 illustrates the application of a voltageacross the second hologram using the voltage generator 15 via theconnectors 17.

In one embodiment of the invention the second holographic recordingmedium is one of a photo thermal refractive or holographic photopolymer,a forward mode HPDLC mixture or a reverse mode HPDLC mixture. In oneembodiment of the invention the third holographic recording medium isone of a photo thermal refractive or photopolymer, a forward mode HPDLCmixture or a reverse mode HPDLC mixture. In one embodiment of theinvention the diffracting thickness of the first hologram is less thanor equal to 1 micron. In one embodiment of the invention the diffractingthickness of the first hologram is less than or equal to 2 micron.

In one embodiment of the invention illustrated in FIGS. 2-3 there isprovided a method of mastering and replicating holograms, the methodcomprising:

a) providing a laser apparatus for forming a first recording beam in afirst direction 1000 and a second recording beam in a second direction1001; a stack 10 of substrates 11-14 each containing a first HPDLCmixture; a holography photopolymer 40 on a substrate 50; a copyholographic substrate 60 containing a second HPDLC mixture 70; a HWP 20;and a linear polarizer 30;b) the first and second beams interfering within each the first HPDLCmixture to form a first hologram in each substrate;c) stacking in sequence the linear polarizer, HWP and first holograms(1-N) onto the second holographic photopolymer;d) illuminating external surface of the holograms (1-N) with light of afirst polarization in the first direction, as indicated by rays1015-1018 in FIG. 4C;e) the first holograms (1-N) diffracting the light into zero order lightin the first direction 1000 and diffracted rays 1011-1014 in FIG. 4C inthe second direction;f) the HWP rotating the incident light polarization through ninetydegrees into a second polarization;g) the linear polarizer removing residual first polarization light;h) the first direction light and the second direction light interferingin the holographic photopolymer to form a second hologram;i) placing the second hologram in contact with the copy holographicsubstrate;j) illuminating external surface of the second hologram with light ofthe second polarization in the first direction 1050;k) the second hologram diffracting the light into zero order light 1051in the first direction and diffracted light in the second direction1052;l) the diffracted and first order interfering in the copy holographicsubstrate to form a third hologram.

In one embodiment of the invention steps d) to l) are repeated for amultiplicity of values of the first and second directions, wherein thefirst and second directions are limited by the diffraction efficiencyangular bandwidth of the first hologram. For example FIG. 4D shows thestep of FIG. 4C in which the first direction is at an opposing angle tothe first direction of FIG. 4C as indicated by the rays 1021,1022. Thisresults in a different diffraction direction as indicated by the rays1030-1033 and 1040-1043. The first direction of FIG. 4D is also used inFIG. 4E.

In the embodiment of FIGS. 2-3 the first polarization is P and thesecond polarization is S. The first hologram is ideally a thin wideangular bandwidth SBG. Thinner grating will provide a broader angularbandwidth. Typically the grating thickness may range from 0.5-1.0microns depending on the angular bandwidth required. The SBGs arerecorded using S-polarized light. The second hologram is recorded into ahigh quality holographic photopolymer material such as the ones suppliedby Bayer Inc. In this case the second hologram is illuminated withP-polarized light as SBGs will only diffract P. To achieve the bestcontrast in the second hologram the exposure must use S-polarized light.This is done by means of the half wave plate (for rotating P to S) andlinear polarizer (for removing residual P-polarised light). Fine tuningof the beam ration is provided by optimizing the angle of the HWPrelative to the linear polarizer. The aim is to have the ratio ofdiffracted to zero order light in the second hologram as close to 1:1 aspossible to provide the optimal beam ratio for the recording of thethird hologram. At this point it will be desirable to increase thesource brightness to compensate for light losses through the polarizingcomponents. As discussed above beam intensity may be fined tune byapplying voltages to the first holograms. The third hologram is createdusing a contact copy process in which the second hologram is illuminatedby S-polarized light

FIG. 7 is a schematic illustration of an optical arrangement using adove prism for recording on of the first holograms in one embodiment ofthe invention. The dove prism 90 with acute angle W (typically 45degree) has its base in contact with the first hologram substrate 10.Construction beams in the directions 1072, 1073 at angles U1, V1 to thesurface normals 1070, 1071 interfere in the substrate 10 to form thefirst hologram.

A method of replicating a hologram according to a preferred embodimentof the invention in accordance with the basic principles of theinvention is shown in the flow diagram in FIG. 8. Referring to the flowdiagram, we see that the method comprises the following steps:

At step 2011 provide: N master SBGs having construction angles in thefirst and second directions; a holographic photopolymer recordingmedium; a HPDLC recording medium; a half wave plate; and a linearpolarizer.At step 2012 stack the master SBGs (1-N), linear polarizer and half waveplate onto the second holographic recording medium.At step 2013 illuminate the master SBGs (1-N) with P-polarized light inthe first direction.At step 2014 master SBGs (1-N) provide 0-order light in the firstdirection and diffracted light in the second direction.At step 2015 the first and second direction light interferes to form thesecond hologram in the holographic photopolymer recording medium.At step 2016 place the second hologram in contact with the HPDLCrecording medium.At step 2017 illuminate the second hologram with S-polarized light inthe first direction.At step 2018 the second hologram provides 0-order light in the firstdirection and diffracted light in the second direction.At step 2019 the first and second direction light interferes to form acopy SBG.

In one embodiment of the invention the first hologram is a surfacerelief hologram such as binary grating. A method of replicating ahologram in one embodiment of the invention in accordance with the basicprinciples of the invention is shown in the flow diagram in FIG. 9.Referring to the flow diagram, we see that the method comprises thefollowing steps:

At step 2021 provide: N substrates each containing a surface reliefhologram having input and diffraction angles in first and seconddirections; a second holographic recording medium; and a thirdholographic recording medium.At step 2022 stack substrates (1-N) onto the second holographicrecording medium.At step 2023 illuminate substrates (1-N) with light in the firstdirection.At step 2024 substrates (1-N) provide 0-order light in the firstdirection and diffracted light in the second direction.At step 2025 the first and second direction light interferes to form thesecond hologram in the second holographic recording medium.At step 2026 place the second hologram in contact with the thirdholographic recording medium.At step 2027 illuminate the second hologram with light in the firstdirection.At step 2028 the second hologram provides 0-order light in the firstdirection and diffracted light in second direction.At step 2029 the first and second direction light interferes to form acopy SBG.

As illustrated in FIGS. 5-6 in one embodiment of the invention there isfurther provided a voltage generator for applied a voltage across atleast one of the second hologram and the third holographic recordingmedium. A method of replicating a hologram in one embodiment of theinvention in accordance with the basic principles of the invention isshown in the flow diagram in FIG. 10. Referring to the flow diagram, wesee that the method comprises the following steps:

At step 2031 provide: N hologram substrates each containing a firsthologram having construction angles in first and second directions; asecond holographic recording medium; a third holographic recordingmedium; and a voltage generator.At step 2032 stack the hologram substrates (1-N) onto the secondholographic recording medium.At step 2033 illuminate the hologram substrates (1-N) with light in thefirst direction.At step 2034 the hologram substrates (1-N) provide 0-order light in thefirst direction and diffracted light in the second direction.At step 2035 the first and second direction light interferes to form thesecond hologram in the second holographic recording medium.At step 2036 place the second hologram in contact with the thirdholographic recording medium and connect the voltage generator to thesecond hologram.At step 2037 illuminate the second hologram with light in the firstdirection. Apply a voltage across the second hologram.At step 2038 the second hologram provides 0-order light in the firstdirection and diffracted light in the second direction.At step 2039 the first and second direction light interferes to form athird hologram.

A further method of replicating a hologram in one embodiment of theinvention in accordance with the basic principles of the invention isshown in the flow diagram in FIG. 11. Referring to the flow diagram, wesee that the method (based on the preferred embodiment discussed aboveand illustrated in FIG. 8) comprises the following steps:

At step 2041 provide: N master SBGs having construction angles in firstand second directions; a holographic photopolymer recording medium; aHPDLC recording medium; a half wave plate; a linear polarizer; and avoltage generator.At step 2042 stack the master SBGs (1-N), linear polarizer and half waveplate onto the second holographic recording medium.At step 2043 illuminate the master SBGs (1-N) with P-polarized light inthe first direction.At step 2044 the master SBGs (1-N) provide 0-order light in the firstdirection and diffracted light in the second direction.At step 2045 the first and second direction light interferes to form thesecond hologram in the holographic photopolymer recording medium.At step 2046 place the second hologram in contact with the HPDLCrecording medium and connect the voltage generator to the secondhologram.At step 2047 illuminate the second hologram with S-polarized light inthe first direction. Apply a voltage across the second hologram.At step 2048 the second hologram provides 0-order light in the firstdirection and diffracted light in the second direction.At step 2049 the first and second direction light interferes to form acopy SBG.

In one embodiment of the invention steps are repeated for a predefinednumber of holographic prescriptions, that is, for a multiplicity ofvectors defining the first and second directions. One first hologram(master) is used to produce all replicas at each prescription. The firstholograms (1-N) are illumination by each first direction vector of apredefined set in turn. The first and second directions are limited bythe diffraction efficiency angular bandwidth of said first hologram.

A method of replicating a hologram in one embodiment of the invention(based on the embodiment of FIG. 1) in accordance with the basicprinciples of the invention is shown in the flow diagram in FIG. 12.Referring to the flow diagram, we see that the method comprises thefollowing steps:

At step 2051 provide N hologram substrates each containing a firsthologram having construction angles in first and second directions; asecond holographic recording medium; and a third holographic recordingmedium.At step 2052 stack hologram substrates 1-N onto the second holographicrecording mediumAt step 2053 illuminate hologram substrates (1-N) with light in thefirst directionAt step 2054 hologram substrates (1-N) provide 0-order light in thefirst direction and diffracted light in the second direction.At step 2055 the first and second direction light interferes to form asecond hologram in the second holographic recording medium.At step 2056 place the second hologram in contact with the thirdholographic recording medium.At step 2057 illuminate the second hologram with light in the firstdirection.At step 2058 the second hologram provides 0-order light in the firstdirection and diffracted light in the second direction.At step 2059 the first and second direction light interferes to form thethird hologram.At step 2060 a new first direction vector is selected from a pre-definedset.At step 2061 the process is repeated from step 2053 onwards until thelast vector in the pre-defined set has been selected.

It should be understood by those skilled in the art that while thepresent invention has been described with reference to exemplaryembodiments, it is to be understood that the invention is not limited tothe disclosed exemplary embodiments. Various modifications,combinations, sub-combinations and alterations may occur depending ondesign requirements and other factors insofar as they are within thescope of the appended claims or the equivalents thereof.

1. A method for mastering and replicating holograms, the methodcomprising: a) providing N substrates each containing a first hologramfor diffracting incident light from a first direction into diffractedlight in a second direction; providing a second holographic recordingmedium; and providing a third holographic recording medium; b) stackingin sequence the first holograms 1-N onto said second holographicrecording medium; c) illuminating external surface of said firsthologram N with light of a first polarization in a first direction; d)said first holograms 1-N diffracting said light into zero order light insaid first direction and diffracted light in said second direction; e)said first direction light and said second direction light interferingin said second holographic recording medium to form a second hologram;f) placing said second hologram in contact with said third holographicrecording medium; g) illuminating external surface of said secondhologram with light in said first direction; h) said second hologramdiffracting said light into zero order light in said first direction anddiffracted light in said second direction; and i) said diffracted andfirst order light interfering in said third holographic recording mediumto form a third hologram.
 2. The method of claim 1 wherein said steps c)to i) are repeated for a multiplicity of values of said first and seconddirections, wherein said first and second directions are limited by thediffraction efficiency angular bandwidth of said first hologram.
 3. Themethod of claim 1 wherein the first holograms 1, N are provided by thesteps of: configuring a laser holographic recording apparatus to form afirst recording beam in said first direction and a second recordingbeams in said second direction; providing N substrates each containing afirst holographic medium; and said first and second beams interferingwithin each said first holographic medium substrate to form said firsthologram in each said substrate.
 4. The method of claim 1 wherein saidfirst holograms 1,N are surface relief diffractive structures.
 5. Themethod of claim 1 further comprising in step a) providing a HWP and instep c) disposing said HWP between said holographic recording mediumsubstrate and said first hologram stack.
 6. The method of claim 1further comprising in step a) providing a linear polarizer and in stepc) disposing said linear polarizer between said HWP and said firsthologram stack.
 7. The method of claim 1 wherein said first holographicrecording medium is a HPDLC for recording a SBG, said second holographicrecording medium is a holographic photopolymer and said third.holographic recording medium is a holographic photopolymer.
 8. Themethod of claim 1 wherein said third hologram is copy of said secondhologram and said second hologram is a copy of said first hologram. 9.The method of claim 1 wherein said third holographic recording mediumcomprises HPDLC material components for forming a forward mode SBG or areverse mode SBG.
 10. The method of claim 1 wherein said zero orderlight and diffracted light in at least one step d) and step i) havepower substantially in the ratio of 1:1.
 11. The method of claim 1wherein said second hologram and said third holographic recording mediumare separated by an air gap.
 12. The method of claim 1 wherein at leastone index matching optical layer is provided.
 13. The method of claim 1wherein said third holographic recording medium forms part of amechanically translatable continuous lamina.
 14. The method of claim 1further comprising a voltage generator for applied a voltage across atleast one of said second hologram and said third holographic recordingmedium; wherein said voltage varies the refractive index modulation ofat least one of said second hologram and said third during steps g) toi).
 15. The method of claim 1 wherein said second holographic recordingmedium is one of a photo thermal refractive or holographic photopolymer,a forward mode HPDLC mixture or a reverse mode HPDLC mixture.
 16. Themethod of claim 1 wherein said third holographic recording medium is oneof a photo thermal refractive or photopolymer, a forward mode HPDLCmixture or a reverse mode HPDLC mixture.
 17. The method of claim 1wherein the diffracting thickness of said first hologram is less than orequal to 2 micron.
 18. A method of mastering and replicating holograms,the method comprising: a) providing a laser apparatus for forming afirst recording beam in a first direction and a second recording beamsin a second direction; N substrates each containing a first HPDLCmixture; a holography photopolymer; a copy holographic substratecontaining a second HPDLC mixture; a HWP; and a linear polarizer; b)said first and second beams interfering within each said first HPDLCmixture to form a first hologram in each said substrate; c) stacking insequence the linear polarizer, HWP and first holograms 1-N onto saidsecond holographic photopolymer; d) illuminating external surface ofsaid hologram N with light of a first polarization in said firstdirection; e) said first holograms 1-N diffracting said light into zeroorder light in said first direction and diffracted light in said seconddirection; f) said HWF rotating the incident light polarization throughninety degrees into a second polarization; g) said polarizer removingresidual first polarization light; h) said first direction light andsaid second direction light interfering in said holographic photopolymerto form a second hologram; i) placing said second hologram in contactwith said copy holographic substrate; j) illuminating external surfaceof said second hologram with light of said second polarization in saidfirst direction; k) said second hologram diffracting said light intozero order light in said first direction and diffracted light in saidsecond direction; and l) said diffracted and first order interfering insaid copy holographic substrate to form a third hologram.
 19. The methodof claim 18 wherein said steps d) to l) are repeated for a multiplicityof values of said first and second directions, wherein said first andsecond directions are limited by the diffraction efficiency angularbandwidth of said first hologram.
 20. The method of claim 18 whereinsaid first polarization is P and said second polarization is S.